Human cystinosis is an inherited disorder characterized by excessive intralysosomal accumulation of the amino acid cystine in certain cells. Although it has been suggested that this disorder may be due to defective lysosomal transport of cystine, experimental efforts to establish this point have hitherto been hampered by the inability to "load" normal cell lysosomes with cystine, either in vitro or in vivo. Recent experiments carried out in collaboration with Drs. J.D. Schulman and R. Steinhertz indicate that such loading can be accomplished by exposure of isolated lysosomes or intact cells to the freely permeable and hydrolyzable cystine dimethyl ester. This technique shows promise in permitting comparative kinetic studies of cystine decay in normal and cystinotic cell lysosomes and, possibly, in detection of heterozygote carriers.